comp-202: foundations of programmingcs202/2016-01/web/sec2/lecture4.pdf · •...

COMP-202: Foundations of Programming

Lecture 4: Methods

Jackie Cheung, Winter 2016

Announcements

Quiz 1 postponed:• Due Jan 26 at 11:59pm

Assignment 1 postponed: • Due on Feb 1 at 11:59pm

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Review

What is the difference between an int and a double?

What is the difference between byte, shorts, ints, and longs?

What is the different between a char and a String?

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This Lecture

Methods• Methods we’ve seen so far

• Using existing Python methods (e.g., String, Math)

• Defining our own methods

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Methods

We’ve been calling certain segments of code methods.public class HelloWorld {

public static void main(String[] args) {

System.out.println("Hello, World!");

}

}

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Calling Methods

We call a method in order to execute the code contained in the method definition.

Some examples?• Every time you run a program, you are calling the

main method of some class of the program.

• System.out.println(<expr>) is a method call!

• Math.ceil(<num>) is a method call!

• Math.log(<num>) is a method call!

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Purposes of Methods

Some methods map an input value to an output value.

e.g., Math.ceil(double a) implements 𝑓(𝑥) = 𝑥 .

Others simply perform some useful action.e.g., System.out.println()prints a new line.

Still others do both of the above (return a value AND perform other actions).

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Using Java Libraries

The JDK comes with many libraries, which contain classes and methods for you to use.

• String library

• Math library

• Swing library (used for graphics)

• Libraries for networking

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Math Library

Contains useful and common methods you can use off-the-shelf (i.e., don't have to write yourself)

Also, constants like E or PI

How do you access them?

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Documentation

Read the Application Programming Interface(API) for the specification of a library.

e.g., Math library: http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html

What information do you get from an API?

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Sample Entry

static double abs(double a)Returns the absolute value of a double value.

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Name of the method

Sample Entry

static double abs(double a)Returns the absolute value of a double value.

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The number and typeof input arguments

The name of the method together with the number and type of input arguments is called the signature of the method.

Sample Entry

static double abs(double a)Returns the absolute value of a double value.

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Description of what themethod does.

Sample Entry

static double abs(double a)Returns the absolute value of a double value.

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The return type of the method

Sample Entry

static double abs(double a)Returns the absolute value of a double value.

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This keyword means this method doesnot have to be called on a particularobject. Don't worry too much about itfor now.

Sample Entry

static double abs(double a)Returns the absolute value of a double value.

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Based on this API entry, we know that wecall use this method with something like:Math.abs(-4.0)

and that this expression evaluates into the value 4.0 with type double.

Sample Entry

static double abs(double a)Returns the absolute value of a double value.

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We also know thatMath.abs("5.0")

would result in an error.

Questions

Give the input arguments (with types), and return types of the following Math library methods, and describe what they do.

Math.pow

Math.random

Math.max

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void Methods

The return type can be void; i.e., these methods do not return a value.

The purpose is to have side effects, not to compute a value directly.

• Note irony of term—side effects are intentional!

• e.g., System.out.println()

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Question

Why doesn’t this make sense?

String a = System.out.println("hi");

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Writing Your Own Methods

You can define your own methods!

Motivation:• Suppose I want to compute the log2 function

from last class, at multiple points in my program.

• I don't want to copy and paste the code into each point where I need it!

• Instead, I write the method once, which I call log2ceil(double x), and call it whenever I need to.

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Adding to Our Toolbox

Analogy: writing a new method is like adding to our toolbox for something we do often.

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How To Write A Method

public static double log2ceil(double x) {

// compute f(x) = ceil(log2(x))

double result = Math.log(x) / Math.log(2.0);

return Math.ceil(result);

}

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Method signature, justlike in the APIKeywords,

return type of double

Curly braces to define method body

General Form

Parts:1. Keywords (e.g., public static)

2. Return type (e.g., void, int, double)

3. Name of method (e.g., log2ceil)

4. Parentheses containing arguments with types

(e.g., double)

5. Body of the method (what it actually does)

Include return statement if return type is not void

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Methods That Return Values

What would the method definition look like for a method that computes the area of a circle?

public static double computeAreaCircle(double r)

What would the method definition look like for a method that compares two Strings and checks if one is the reverse of another?

public static boolean checkIfReversed(String s1,

String s2)

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Using Methods That Return Values

Since these methods return values (and may also have side effects), we can use the result as part of expressions.

// sum areas of two circles

double sumArea = computeAreaCircle(5.0) +

computeAreaCircle(4.0);

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How To Return Values

return <expression>;

e.g., A pretty useless method:public static int identity(int a) {

return a; // just return a itself

}

The expression returned must match what is expected in the header of the method.

Execution returns immediately to where the method was called.

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return Statement

public static double foo(double a) {

return a * 2;

return a * 3;

}

Second return statement is unreachable.

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Printing vs. Returning

Be careful of the difference:Write a method that prints the sum of three doubles

Write a method that returns the sum of three doubles

What would be the return type of the two methods?

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Tracing the Code

Let’s use our newly defined method multiple times, and trace the execution of the code!

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Trace this Codepublic class TraceLog2 {

public static void main(String[] args) {

System.out.println(log2ceil(3.0));

System.out.println(log2ceil(6.0));

System.out.println(log2ceil(9.0));

}

public static double log2ceil(double x) {

// compute f(x) = ceil(log2(x))

double result = Math.log(x) / Math.log(2.0);

return Math.ceil(result);

}

}

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Example: computeAreaCircle

Fill in the method body of computeAreaCircleLet's do this together.

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Exercise

Write a method called sayGreeting():it takes two Strings as input,

the first represents the speaker,

the second represents the listener,

it prints a String saying the speaker saying something to the listener, like "<Speaker> says hello to <Listener>"

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Methods Calling Methods

Methods can call other methods.e.g., Let’s implement a method that returns the volume of a sphere given its radius, and implement our own cube method!

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Exercise: Die Roll

Write a method that returns a double, which is one of 1.0, 2.0, 3.0, 4.0, 5.0, or 6.0, each selected with equal probability. This corresponds to rolling a six-sided die.

• What is the return type?

• What is the list of arguments?

• Test your method by calling it in the main method.

• Use methods in the Math library to help you.

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